In recent years a great deal of interest has been shown in the provision of a means to enable the deaf to hear without use of their ears.
In my U.S. Pat. Nos. 4,052,572 issued Oct. 4, 1977 and 4,220,830 issued Sept. 2, 1980 there are disclosed two particular forms of hearing aid that I have called a "cortical hearing aid".
For best results with the profoundly deaf, the cortical hearing aid described in my U.S. Pat. No. 4,052,572 generally requires a radio frequency carrier of fixed frequency somewhere in the range between 30 KHz and 100 KHz. This carrier is amplitude modulated by the audio-frequency signals that convey the desired information to the deaf person.
As noted in my U.S. Pat. No. 4,220,830 a carrier signal may be modulated to produce a suppressed-carrier modulated signal. In a cortical hearing aid of this type, it is of great importance to use a carrier whose frequency is extremely low as compared to the amplitude modulated signals used in radio broadcast transmission. A balanced bridge modulator (14) is utilized. This is because the upper limit (e.g. 10 KHz) of the audio-frequency spectrum used as modulation approaches the lower range of the optimum carrier-frequency range. Thus, in accordance with my U.S. Pat. No. 4,052,572 the modulation frequency bandwidth is limited to approximately 7 KHz or lower to prevent distortion. This is because the capacitors used to stabilize the operation of the modulator shunts the higher audio frequencies to ground.
Other prior patents of interest include U.S. Pat. Nos. 2,373,569 issued 4/1945 to Kannenberg; 3,238,472 issued 3/1966 to Crompton-Couvela; 3,393,279 issued 7/1968 to Flanagan; 3,514,720 issued 5/1970 to Roucache et al; 3,811,098 issued 5/1974 to Williams; 4,052,572 issued 10/1977 to Schafer, and 2,703,344 issued 3/1955 to Anderson.
These patents are mentioned as being representative of the prior art and other pertinent references may exist. None of the above cited patents are deemed to affect the patentability of the present claimed invention.
The present invention involves a novel combination of features and circuitry combined in such a way as to afford a very efficient, cost effective, solution to the difficulties and problems encountered with the prior art.
The use of modulating frequencies as high as 20 KHz is desirable for most accurate discrimination of speech and the fullest enjoyment of music.
The present invention resides in part on the discovery that an unbalanced bridge amplitude modulator may be utilized in a cortical hearing aid to provide improved bandwidth (e.g. up to 20 KHz) of the modulated audio signal; generally eliminates sensitivity to destabilization with electrode length variation; eliminates any problem associated with input (audio) frequency doubling; improves uniformity of bandwidth at different proportions of modulation levels and appears to provide improved signal fidelity over an increased bandwidth range.
The Cortical Hearing Aid is a devioe designed to produce the sensation of hearing by electrical stimulation. Electroacoustic transducers or electrode implants (or any other form of surgery) are not required. The carrier (about 40 KHz) is modulated by the amplified output of a microphone or audio oscillator, and this modulated signal is applied to the patient through two electrodes: (1) a bare metal electrode (indifferent electrode) which is electrically at ground potential and may be placed anywhere on the patient's body; (2) the other is an insulated electrode (stimulus electrode) which is lightly held against the most effective area of the patient's head. This area must be determined by experiment, and does vary somewhat from patient to patient. The placement of the stimulus electrode determines the most effective "point of entry" for the signal.
In effect, the electronic circuitry scans the neurons at a rate of 40,000 times per second and thus is the electrical equivalent of several thousand electrodes. It operates on a constant current basis, so that current flow is largely independent of the electrical impedance of the patient's head. This frequency is high enough so there is no possibility of interference with "brain waves" (Alpha, 8 to 13 Hz; Beta, 14 to 30 Hz; Theta, 4 to 7 Hz; Delta, 0.5 to 3.5 Hz), or pacemaker (heart) pulses. At the same time, the 40 KHz frequency is low enough so that it is out of the range of those frequencies producing overheating of internal body organs (150 to 1250 mHz) or cataracts in the eyes (100 to 10,000 mHz).